Engine lubricating oils require the presence of additives to protect the engine from wear. For almost 40 years, the principal antiwear additive for engine lubricating oils has been zinc dialkyl dithiophosphate (ZDDP). However, ZDDP is typically used in the lubricating oil at a sufficient concentration to provide a phosphorus content of 0.12% by weight or higher in order to pass required industry standard tests for antiwear. Since phosphates may result in the deactivation of emission control catalysts used in automotive exhaust systems, a reduction in the amount of phosphorus-containing additives (e.g., ZDDP) in the oil would be desirable. The problem sought to be overcome is to provide for a reduction in the amount of phosphorus-containing additive in the lubricating oil and yet provide the lubricating oil with desired antiwear properties.
The use of metal salts of phosphorodithioic acids as additives for lubricants is disclosed in U.S. Pat. Nos. 4,263,150; 4,289,635; 4,308,154; 4,322,479; and 4,417,990. Amine salts of such acids are disclosed as being useful as additives for grease compositions in U.S. Pat. No. 5,256,321.
U.S. Pat. No. 4,758,362 discloses the addition of a carbamate to a low phosphorus or phosphorus free lubricating oil composition to provide such a composition with enhanced extreme-pressure and antiwear properties.
U.S. Pat. No. 5,034,141 discloses that improved antiwear results can be obtained by combining a thiodixanthogen (e.g., octylthiodixanthogen) with a metal thiophosphate (e.g., ZDDP). U.S. Pat. No. 5,034,142 discloses the addition of a metal alkoxyalkylxanthate (e.g., nickel ethoxyethylxanthate), a dixanthogen (e.g., diethoxyethyl dixanthogen) and a metal thiophosphate (e.g., ZDDP) to a lubricant to improve antiwear.